1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to circuit interrupters including a number of current transformers. The invention also relates to current transformers. The invention further relates to methods of manufacturing current transformers.
2. Background Information
Molded case circuit breakers (MCCBs) employ current transformers (CTs) to sense primary currents and to supply power to an electronic trip unit. Currents are sensed and the resulting sensed current values are typically used in two modes. In a normal mode, the range of current values is used to calculate trip times having intentional delays. In an override mode, the current values are higher than the normal mode current range and the corresponding trip time is instantaneous. The override mode is responsible for clearing relatively high current faults in the shortest possible time with no intentional delay. In order to be able to properly respond to a high current alternating current (AC) fault, the fault current must be sensed during its first half-cycle. However, known CTs cannot sense current accurately during this first half-cycle due to transformer core design limitations.
One of these limitations is the inability of the CT core to handle extreme changes in magnetic flux. The CT core is constructed of ferromagnetic materials, which tend to “saturate” at a predetermined level that is dictated by, for example, the core material and the core dimensions. The saturation point is where any further increase in magnetizing field force (H) does not result in a proportional increase in magnetic flux density (B) or any more increase in the secondary current resulting from this magnetic flux change. Consequently, the CT secondary waveform is highly distorted and of relatively low magnitude.
The first AC half-cycle of the CT output is dependent on the polarity of the previous AC half-cycle. Referring to FIG. 1, if the previous AC half-cycle was positive when the magnetizing field force (H) was removed, then the CT core will retain a residual or remanent magnetization referred to as +Br. The magnitude of +Br is dependent on the magnetizing field force (+H) and the type of core material.
If the next AC half-cycle is negative, then the magnetic flux density (B) can swing from approximately +Br to −Bmax depending on the magnitude of the corresponding magnetizing field force (−H). This swing will cause a relatively large ΔB (i.e., +Br−(−Bmax)=+Br+Bmax) and, therefore, a relatively large ΔI in the CT secondary current. However, if the next AC half-cycle is positive, then the magnetic flux density (B) can only swing from +Br to +Bmax. In known MCCBs, this will be a relatively small change in flux and, therefore, a resulting relatively small change in CT secondary current. Also, the CT core will reach saturation much faster now since Br is closer to +Bmax. In other words, ΔB will be much smaller in this case (i.e., +Bmax−(+Br)=+Bmax−Br) and the resulting secondary current also will be smaller in magnitude.
Accordingly, there is room for improvement in circuit interrupters.
There is also room for improvement in current transformers.
There is further room for improvement in methods of manufacturing current transformers.